The present invention relates to a transmission power control system in a radio apparatus for controlling transmission power on the transmission side of a radio communication system.
Generally speaking, in a radio communication system for transmitting signals in free space, there is a possibility that the radio communication state will deteriorate due to fading caused in a space or due to interference from other radio communication systems. Therefore, when the propagation state is good, the transmission power is lowered to decrease interference with other systems. However, when the propagation state is bad, the transmission power is raised. Therefore, without causing an interference, the fading problem is solved and the transmission power is controlled in accordance with a receiving level data signal.
FIG. 1 shows a block diagram of the transmission power control system in a general wireless apparatus. The system comprises a transmitter 1 (transmitting unit), a transmission antenna 2 for transmitting a transmission signal from transmitter 1, a receiving antenna 3 for receiving a signal from transmission antenna 2, a receiver 4 (receiving side), a level detecting process circuit 5 for detecting the receiving level of the receiver 4 and for transmitting the receiving level to the transmission side, and a transmission control circuit 6 for receiving receiving level information from the level detecting process circuit 5 and for controlling the transmission power of the transmitter 1.
Next, transmitter 1 and a conventional transmission power control system will be explained in detail by referring to the first prior art example shown in FIG. 2. Transmitter 1 comprises a variable attenuator (VATT) 1a for adjusting an intermediate wave (IF) signal level, a distortion compensator (PD) 1b for compensating the distortion of the output of the high output amplifier 1e, an up-converter 1c for converting the output of the distortion compensator 1b to a signal of higher frequency, a high power amplifier (HPA) 1e, as mentioned above, for amplifying the output of up-converter 1c and outputting it to transmission antenna 2, and local oscillator 1f for an up-converter.
Transmission control circuit 6 controls the transmission power by controlling variable attenuator 1a based on a detection signal having an output level obtained from high output amplifier 1e and based on receiving level information obtained from a level detecting process circuit 5. However, the transmission power control system shown in FIG. 2 may sometime produce distortion because the level relation between distortion compensator 1b and high power amplifier 1e changes when the gain change of the high output amplifier 1e is corrected by the intermediate frequency signal side.
Another prior art transmission power control apparatus is shown in FIG. 3. In the second prior art shown in FIG. 3 a second variable attenuator 1d (when attenuator 1a is called the first variable attenuator) is inserted between up-converter 1c and high output amplifier 1e. The second variable attenuator 1b is controlled by comparator 6a. Comparator 6a receives a detecting signal with output level information obtained from high output amplifier 1e and a previously set reference voltage, and then controls the second variable attenuator 1d based on the difference between the detecting signal and a reference voltage. The apparatus forms a transmission control unit by using transmission control circuit 6, which is the same as the apparatus shown in FIG. 2, and the above-described comparator 6a.
With this structure, the transmission power determined by the receiving level information is controlled by the first variable attenuator 1a and the gain variable portion of high output amplifier 1e is corrected by the second variable attenuator 1d.
However, the prior art transmission power control system shown in FIG. 3 comprises a controlling system with a double loop structure, which makes the structure and control complicated.